And on top of that going with a Motorola processor that was probably top of the line in 80s but was well past its sell by date by 1993
But you can avoid the M68K for rendering. Of course that wouldn't go anywhere in terms of 3D performance. The main processor of Jaguar is not really the CPU, but Tom (GPU + memory controller + objects processor + blitter).
OpenLara on GBA is an amazing achievement, but I suppose it is not "difficult" by itself. Lots of studios did 3D games back in the day. The console itself is pretty straightforward when it comes to CPU, memory and rendering. While Jaguar is a big pita due number of processors, lacking of processors cache, memory layout, etc.
Anyway, the current OpenLara on GBA with outstanding performance and two levels is relatively recent, like two years, but the original port has been in development for many years, I remember the travelling tech demo (with far worse performance) like 10 years ago. So I guess he has been working on it for a long time. Checking the Github, it seems he has been working on 32X and 3DO ports, but the Jaguar port is probably abandoned. There are also two-three N64 ports out there, but handled by another people.